Thermal coal upgrading process

ABSTRACT

A process for thermally upgrading coal in a vertical processor wherein the coal is gravity fed and a heated inert gas is introduced into the processor. The temperature of the gas, the size of the coal and the rate of movement of the coal are controlled to efficiently remove moisture from the coal and to not remove volatile organic compound.

CROSS REFERENCE TO RELATED APPLICATION

The present application is related to provisional patent applicationSer. No. 60/759,513 filed Jan. 17, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention is an improved thermal coal upgrading processwhich efficiently treats higher moisture content coals (and othersimilar materials but herein referred to as coal) with relatively lowgrade (“waste”) heat sources from an interconnected host facility. Thesubject invention involves a solids size separation means, a solids togas contacting means, a means of controlling the solids discharge fromthe solid to gas contacting means, a fine solid feeding means and a hotsolids and fines solids mixing means.

2. Background of the Invention

Due to the inventors' extensive history with and knowledge of theSynCoal® process this invention is compared to the original SynCoalProcess for explanatory purposes. ( SynCoal® is a registered trademarkfor both the process and the product that results from the processalthough the ® is not generally included herein for brevity.) Theoriginal SynCoal process was patented by Monroe Greene, U.S. Pat. No.4,725,337 issued Feb. 16, 1988 and U.S. Pat. No. 4,810,258 issued May 7,1989. The demonstration SynCoal facility operated throughout the 1990sand received coal sized to accommodate the material handling componentsof this demonstration facility. Coal was screened to eliminate materiallarger than 1½ inch and reduce the amount of material smaller than ½inch. The oversized and undersized material was returned to the minestockpile, a situation that cannot be sustained on a large commercialbasis either at a mine site, or at an end consumer's site withoutsignificant efforts to mix untreated coal with the upgraded coal.

The gas to solids contacting components selected for the originalSynCoal facility for treating the coal were vibrating, fluidized bed(VFB) processors (aka conveyor dryers) requiring relatively highdifferential pressure process gas fans. While the VFBs provide aneffective mechanism for contacting the coal with the process gas, theyproved to be difficult to maintain due to excessive thermal andmechanical stresses and resulted in removal of significant quantities offine material from the coal and consumed large amounts of electric powerfor the high pressure process gas fans. Additionally, subsequent effortsto develop larger facilities found it was nearly impossible to expandthe productive capacity of the individual VFB processor units.

The method for cooling the upgraded coal in the original application wasineffective and expensive. The first stage of cooling used a water sprayintended to flash cool the coal. The second stage used a VFB to contactthe upgraded coal with a cool, saturated gas. The entire systemdelivered an upgraded product at about 180° F., well above the safetemperature limit for storage and required continuous inerting toprevent product spontaneous combustion. These limitations led toextensive efforts to identify better equipment and process options. Thecooled, dried coal was screened with each sized fraction delivered toindividual vibratory gravity separation systems known as air tables orjigs. The particular gravity separation components were prone tomechanical failures and did not allow for unit capacity increases forlarger volume applications.

The inventors are aware of the following prior art:

5,137,539 Bowling Aug. 11, 1992 4,043,763 Norman Jan. 07, 1977 4,750,913Siddoway Dec. 19, 1986

Bowling '539 teaches drying coal rapidly in a fluid bed process usingcombustion gases from a steam generation system where the finer sizeddried coal is combusted in the steam generation system and the coarsersized dried coal is meant to be transported (presumably in conventionalbulk transport and handling systems) to other users. Actual commercialfield experience has taught the present inventors that this process willresult in a dusty, low density product that is susceptible tospontaneous combustion, making conventional handling of the dried coarsecoal product problematic. Bowling does not teach process residencetimes, heating rates, contact velocities or operating pressures.Bowling's focus upon rapid drying, making a transportable product, lackof heating rate or contact velocities are distinct departures from thesubject invention.

Norman '763 teaches mixing warm dried fine coal (0.5 to 10.0% moisture @175°-480° F. (80° to 250° C.), presumably produced with a rapid heatingrate process with coarse run of mine coal (25 to 50% moisture @ ambient)to rehydrate and cool the dried coal, producing a stable (passivated)mixture. The ratio of dried coal to raw coal can vary from 1:1 to 1:10,depending on the moisture content and temperature of the dried coal. Theobjective of this patent is to stabilize dried coal for storage andhandling. Actual commercial field experience has taught the presentinventors that mixing dried and undried coal produces an extremely dustyproduct, making conventional handling of the mixed coal problematic. Thepresent invention slowly heats coarse coal particles and then mixes themwith fine coal reducing the moisture content and producing an upgradedcoal product that can be physically cleaned with no intention of makinga product that will be handled with conventional bulk handling orstorage techniques resulting in distinct differences between theprocesses.

Siddoway '913 teaches a rapid drying and cooling process using fluidizedbed technology. A portion of the process feed coal is rapidly dried in afluidized bed dryer and then mixed with the balance of the feed to theprocess. The mixture is then fed to a fluidized bed cooler. Preferably,the finer material is fed to the fluidized bed dryer and the coarsermaterial mixed with the dried coal prior to delivery to the fluidizedbed cooler. Fines collection and distribution is discussed together withprocess gas treatment. Of particular interest is the use of raw coal toassist in the cooling of the dried coal. Convective and evaporativecooling is discussed, but in the context of the application to fluidizedbed technology. The desired goal is to produce a product more resistantto spontaneous combustion by reducing the temperature of the driedproduct. Mixing raw coal with dried coal is one of the process steps inthe process of the present invention. The difference is that the presentinvention process maximizes gas contact time, minimizes contact gasvelocity which minimizes compressive energy input. Additionally, thepresent invention teaches the use of fines as the cooling medium asopposed to the use of coarse material and cool gas as taught bySiddoway. The present invention produces a product temperature of 140°F., while the reference predicts a product temperature of less than 100°F. Rapid fluid bed drying only the fine coal fraction and the highercontact gas velocities are distinct departures from the process of thepresent invention.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved thermalcoal upgrading process to efficiently remove moisture from coal withoutremoving the volatile organic compound (VOC).

It is a further object of the present invention to provide an improvedthermal coal upgrading process which is economical.

In accordance with the teachings of the present invention, there isdisclosed a process for thermally upgrading the coal The coal isseparated into coarse and fine fractions. A vertical processor isprovided having a plurality of baffles therein. The coarse fraction ofcoal is introduced into the top of the processor wherein the coarse coalmoves to the bottom of the processor by gravity. A moderately hightemperature process gas is provided. The process gas is introduced intothe processor under rows of baffles where the gas is uniformlydistributed at a low velocity in a cross flow manner throughout the coalin the processor. The coal flows by gravity around the baffles gentlyagitating the coal and the process gas heats the coal to remove moisturefrom the coal which alters the combustion and physical characteristicsof the coal. The heated, altered coal is removed from the processor. Theheated altered coal is mixed with a controlled amount of the finefraction coal wherein the coarse fraction of the coal is cooled and thefine fraction of the coal is heated.

These and other objects of the present invention will become apparentfrom a reading of the following specification taken in conjunction withthe enclosed drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of the process of the present invention.

FIG. 2 is a flowchart of a power boiler integrated with the process ofthe present invention.

FIG. 3 is a graph showing mercury associated with pyrite in coal.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Generally, all the relevant prior art is focused upon creating a driedcoal that has bulk material characteristics that allow conventional bulktransportation in open top rail cars and trucks and storage in openstockpiles. Additionally, the relevant prior art processed a selectedcoarse fraction of the feedstock requiring alternative use of thenon-selected finer sized material. The subject invention does notattempt to achieve these characteristics but focuses upon processing theentire range of feed coal size at a slow heating rate with low contactgas velocity resulting in minimal fines elutriation and volatile organiccompound evolution (loss) while maintaining the granular productcharacteristic that is amenable to physical cleaning. Based upon theirextensive commercial field experience, the subject inventors do notexpect to be able to handle the upgraded coal product in conventionalrailcars, open storage piles or high capacity bulk handling systems.

In the present invention, coal has been previously crushed to eliminateoversized material larger than approximately three inches. It ispreferred that the coal not exceed two inches. The preferred processordesign maximum feedstock size specifications is two inch by zero. Ascreen is used to divert a portion of the finer coal for use in thecooling process. Reducing the fines in the thermal process will reduceparticulates in the exit gas stream and reduce the power required toforce the process gas through the upgrading processor system, andoptimized the cooling of the dried and altered coal.

An improved tower processor is used to gently mix the coal particleswhile exposing them to low velocity, moderately high temperature processgas in a cross flow manner that results in small pressure differentialsbetween the process gas inlets and outlets thereby minimizing thecompressive power requirements. As the coal dries and descends throughthe processor, the coal particles will break up by thermal forces andattrition, shifting the size distribution of the coal to a smalleraverage particle size. The tower processor is crossed with alternatinglevels of inverted V inlet and outlet baffles extending the full widthof the tower.

The coal is treated using a moderately high temperature, inert processgas in intimate contact with the coal. The coal to gas contact time canbe varied widely, generally from 20 minutes to several hours but isidealized to be about 60 minutes versus the 10 to 15 minute contact timein the original application. Although a relatively wide range oftemperatures (450° F. to 900° F.) and “inert” gas compositions(combustion flue gas to steam) can be applied, ideally inlet gastemperatures would be attemperated to approximately 700° F. and oxygencontent maintained as low as possible within reasonable margins ofsafety, to minimize or prevent devolatilizing the coal while providingthe process heat required. Preferably, the oxygen content is less than5% by volume. The higher the difference between the inlet and outletprocess gas temperature, the smaller the volume that is needed to heatthe coal and the lower the gas velocities resulting in less finematerial carryover and lower process gas compressive power required.This relationship can be described as the gas to coal ratio. In thepreferred embodiment, as the coal is heated and the moisture vaporizedinto the process gas stream (make gas), the combined process and makegas temperature is about 225° F. at the processor exit. Using thecurrent invention, coal in the processor is heated at a rate of not toexceed 10° F. and preferably less than 3° F. per minute resulting inmore thorough upgrading without creating significant quantities ofvolatile organic compounds (VOCs). Empirical testing (shown in Table 1)by the inventors has shown that VOCs evolved by this process arenegligible as long as the moisture content of the coal is not reduced toless than 1.5%. Reduction to lower moisture contents results in theproduction of very small quantities of VOCs.

TABLE 1 Test No. 1 2 3 Process Gas Temperature ° F. 600 650 700 NormalProcessing Outlet 300 312 351 Gas Temperature ° F. Process Time; Minutes62 60 60 Coal Moisture 2.5% 1.5% 0.5% VOCs in Exhaust Gas ppm 0 0 10Extended Processing Outlet 309 321 361 Gas Temperature ° F. ProcessTime; Minutes 77 75 75 Coal Moisture 1.8% 0.1% 0.0% VOCs in Exhaust Gasppm 0 0 30 Final Coal Temperature ° F. 305 318 347

Within these temperature ranges, a variety of fuel combustion processes,external sources such as steam boilers, cement/lime kilns, hydrocarbonrefining processes, and coal liquefaction and gasification processes canprovide “waste” heat and/or combustion gas to the improved process whichcan in turn provide an improved solid fuel feedstock to theseoperations. Additionally any steam condensation or water coolingrequirements of the host facilities can be integrated into the top ofthe thermal processor by placing condensing tubes in the baffles of theprocessor to improve the overall thermal efficiency of the process. Theprocess gas may be reheated by indirect exchange with a waste heatsource and recycled to the processor.

The heat energy that is required to be transferred from the process gasto the coal is dictated by the amount of moisture to be removed. The gasflow rate (velocity) is determined by coal's residence time in theprocessor and the volume of process gas required per unit of coal to beprocessed. The long residence and slow rate of heating results in moreuniform and complete upgrading without producing significant quantitiesof VOCs.

The process gas velocities are extremely low when compared to otherprocessor operations. This minimizes the size of the particle that willbe removed from the coal being processed and the process gas fan powerrequired (see Table 2). The power required to drive the gas through thesubject process is nearly an order of magnitude less than that requiredfor the original VFB operation. The VFB's in the original facility hadbed velocities of approximately 10-12 fps and combined with thevibratory bed action carried particles as large as 0.0234 inches (˜28mesh) into the exit duct work where the higher velocities entrained theparticles requiring the need for extensive dust collection equipment.This system resulted in a 45 inch water column (iwc) total systemdifferential pressure. The current invention system design has particleelutriation velocities less than 5 fps, preferably, less than 2 wherethe process gas exits the coal bed and a total system pressure drop ofless than 5 iwc. At this low elutriation velocity, coal particles largerthan 0.0035 inch (170 mesh) should not be entrained and carried out ofthe coal bed.

Stokes Law defines an expression where particles falling in a viscousfluid by their own weight teach a point where the frictional forcecombined with the buoyant force exactly balance the gravitational forcetermed the terminal or settling velocity. This velocity translatesdirectly into the elutriation velocity.

Centrifugal fan horsepower is a function of the pressure drop and themass flow through the gas flow path. The reduction in system pressurerequirement reduces process gas fan power requirements by 9 times overthe previous experience. Additionally, for any fixed flow path, pressuredrop is a function of the square of the gas velocity and mass flow is afunction of the gas velocity and the area of the flow path. Therefore,for a fixed flow path and gas density, fan horsepower requirements are adirect function of the cube of the velocity.

TABLE 2 Stokes Law Relative Power Exit Gas Velocity Max Particle SizeRequired 1.0 fps 0.0040 1 3.0 0.0061 27 6.0 0.0086 216 11.0 0.0117 1,331

The fine coal removed prior to the processor feed is mixed with thethermally treated product, cooling the hot coal while removing some ofthe moisture from the fine coal, resulting in a lower temperatureproduct than was attained using the combination of quench and VFBmechanisms. The fine coal can be fed to the mixing system through astand pipe that allows excess fines to overflow into the processor feedbin so that the fine coal used for cooling is always in controlledproportion to the processor product. The mixing process can beaccomplished using a screw auger or pug mill where hot coal product fromthe processor is mixed with the coal fines. The combination of sensibleand latent heat can reduce the product to a more acceptable temperaturetypically less than 180° F. and preferably approximately 140° F. Thefinal product temperature can be varied with different ratios of finecoal to processor product.

Heating the coarser coal the processor, and then mixing the dried,altered coal with the raw coal fines heat the fine particles whilecooling the dried, altered coal. This cooled product has virtually thesame physical and chemical characteristics as the original SynCoal priorto physical coal cleaning. As a result, similar to the original SynCoal,the product has enhanced cleaning characteristics and can be fed to acommercial dry gravity separation process to reduce mineral componentsof the feed coal further improving the product's combustion andenvironmental characteristics.

The improved process of the present invention produces a nearlyidentical final product (but at a safer lower temperature) as comparedto the original process. The final product of the present inventionprovides improved combustion efficiency, boiler capacity and reducedenvironmental emissions as shown previously. Many heavy metalcontaminants (such as mercury) that form air toxic emissions have beenfound to be associated with pyrite minerals (FIG. 3). Pyrite mineral canbe removed from the upgraded coal more effectively than from the rawcoal as has been previously demonstrated, especially with the largersized particles. The association of these heavy metals with the pyritemeans that cleaning the upgraded coal can reduce the amount of air toxicemissions that are released when combusting the cleaned upgraded coal.

From a side by side test with two nearly identical commercial powerplants—burning a raw Montana Powder River Basin (NPRB) coal and theother blending at average of 12.3% SynCoal with the same raw NPRB coalover a three month period, the following results were observed.

TABLE 3 12.3% SynCoal NPRB Coal Blend Gross Power, MW 302.6 310.2 AuxPower, MW 24.6 23.3 Net Power, MW 277.9 286.9 Net Plant Heat Rate 11,25910,998 SO2 lbs. per MM Btu 1.744 1.686 SO2 Emission Rate, lbs. per MMBtu 0.432 0.388 NOx Emission Rate, lbs. per MM Btu 0.424 0.313

Laboratory mineral separation testing on a typical Montana NPRBsub-bituminous coal demonstrated that mercury content is associated withpyrite content.

The ash, sulfur, mercury and air toxic removal shown by the laboratorytesting is as follows:

TABLE 4 NPRB Coal SynCoal Reduction Lbs. Ash per MM Btu 9.80 7.73 21%Lbs. SO2 per MM Btu 1.37 0.91 34% Lbs. Hg per T Btu 7.00 3.06 56% Lbs.Air Toxic per MM Btu 0.21 0.19  9%

The process of the present invention is simplified, minimizingmaintenance, energy use and operator interface dramatically reducingoperating staffing and costs compared to the original application. Inthe preferred embodiment, the integrated application of the improvedprocess at a thermal host provides for the use of “waste” heat andminimizes capital and operating energy costs providing significanteconomic advantage to this application of the process over the currentcompetitors.

Obviously, many modifications may be made without departing from thebasic spirit of the present invention. Accordingly, it will beappreciated by those skilled in the art that within the scope of theappended claims, the invention may be practiced other than has beenspecifically described herein.

1. A process for thermally upgrading coal comprising the steps of:separating the coal into coarse and fine fractions, providing a verticalprocessor having a plurality of baffles therein, introducing the coarsefraction of coal into the top of the processor wherein the coarse coalmoves to the bottom of the processor by gravity, providing moderatelyhigh temperature process gas, introducing the process gas into thebottom of the processor under rows of baffles wherein the gas isuniformly distributed at a low velocity in a cross flow mannerthroughout the coal in the processor, the coal flows by gravity aroundthe baffles gently agitating the coal while the direct contact with themoderately high temperature process gas heats the coal to removemoisture from the coal altering the combustion and physicalcharacteristics of the coal, removing the heated, altered coal from theprocessor, and mixing the heated altered coal with a controlled amountof the fine fraction coal wherein the coarse fraction of the coal iscooled and the fine fraction of the coal is heated releasing some of themoisture contained in the fine fraction.
 2. The process of claim 1,wherein the coarse fraction of the coal does not exceed approximatelythree inches.
 3. The process of claim 1, wherein the baffles arealternating levels of inverted V inlet baffles and outlet bafflesextending transversely across the processor wherein the coal is gentlymixed as the coal descends by gravity through the processor reducingcompressive power requirements.
 4. The process of claim 1, wherein theprocess gas is heated by waste heat from an external source.
 5. Theprocess of claim 4, wherein the process gas is exhaust gas from acombustion process.
 6. The process of claim 4, wherein the process gasis reheated by indirect exchange with a waste heat source and recycledto the processor.
 7. The process of claim 1, wherein the temperature ofthe process gas ranges from 450° F. to 900° F.
 8. The process of claim7, wherein the temperature of the process gas is approximately 700° F.9. The process of claim 1, wherein the oxygen content of the process gasis less than 5% by volume.
 10. The process of claim 1, wherein the coalin the processor is heated at a rate of less than 10° F. per minute. 11.The process of claim 10, wherein the coal in the processor is heated ata rate of less than 3° F. per minute.
 12. The process of claim 1,wherein a minimum amount of volatile organic compounds (VOC) are evolvedfrom the coal.
 13. The process of claim 1, wherein the process gas exitsthe coal at a velocity of less than 5 fps.
 14. The process of claim 13,wherein the process gas exits the coal at a velocity of less than 2 fps.15. The process of claim 1, wherein the product has a temperature ofless than 180° F.
 16. The process of claim 15, wherein the product has atemperature of less than 140° F.
 17. The process of claim 1, wherein theresidence time of the coal in the processor is approximately 60 minutes.18. A continuous process for thermally upgrading low rank coalcomprising the steps of: providing a tower means for passing the coalvia gravity from the top of the tower, the tower having baffle meanswith the tower for channeling the coal downwardly within the tower,providing a source of heated gas, and introducing the heated gas inoutlet means within the tower wherein the heated gas mixes within thetower heating the downwardly descending coal to remove moisture from thecoal, the heated gas passing through outlets in the tower.
 19. Theprocess of claim 18, wherein the temperature of the heated gas rangesfrom 450° F. to 900° F.
 20. The process of claim 18, wherein the coal inthe processor is heated at a rate of less than 3° F. per minute.
 21. Theprocess of claim 18, wherein the residence time of the coal in theprocessor is approximately 60 minutes.